1. Field of the Invention
This invention relates to forming images of joint structures, and, more particularly, to forming such images as body members connected to the joint structures are moved.
2. Summary of the Background Information
Magnetic resonance imaging (MRI), which is additionally called nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT), is a medical imaging technique used to visualize internal structures of the body, making use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body in a process known to create more detailed images of the human body than possible with X-rays. A conventional MRI scanner includes a large, powerful magnet, extending around a cavity in which the patient 108 lies, while a magnetic field formed by the magnet aligns the magnetization of certain atomic nuclei within the body, and while radio frequency magnetic fields are applied to systematically alter the alignment of this magnetization. This causes the nuclei within certain atoms in the body to produce a rotating magnetic field detectable by the scanner, which in turn causes information to be recorded for constructing an image of the scanned area of the body. Magnetic field gradients cause nuclei at different locations to process at different speeds, allowing spatial information to be recovered using Fourier analysis of the measured signal. By using gradients in different directions, two-dimensional images or three-dimensional volumes can be obtained in any arbitrary orientation. The resulting images display good contrast between the different soft tissues of the body, making the MRI process especially useful in imaging the brain, muscles, heart, and cancers compared with other medical imaging techniques such as computed tomography (CT) or X-rays. Furthermore, an important advantage for MRI arises from the fact that unlike CT scans or traditional X-rays, MRI does not expose the patient to ionizing radiation.
For some time it has been possible to make moving images of the heart and other organs from MRI scans, with such images being displayed, for example, on the Internet. However, in the past, such images were reconstructed after completion of the scanning process by combining images derived from a number of repetitive actions (such as a heart beat occurring while the subject holds his breath) into a motion picture sequence.
More recently, a process, known as “Real-time MRI,” has been developed for making pictures of moving objects using MRI in real time, so that the images are displayed during the scanning. Because the generation of image data was based on a time-consuming process of data reconstruction from scanned data, in what is known as ‘k-space,” real-time MRI was formerly possible only with low image quality or low temporal resolution. In the past such images could only be formed at a rate of about one per second.
This limitation has now been eliminated, particularly through the use of an iterative reconstruction algorithm that alleviates problems caused by undersampling, and with radial FLASH MRI, in an improved process offering rapid and continuous data acquisition, motion robustness, and tolerance to undersampling. In particular, improved techniques integrates the data from multiple receive coils, in a process also known as parallel MRI, and exploits the redundancy in a sequence of images, reducing the effects of data undersampling by an order of magnitude, so that high-quality images may be obtained from of as little as 5 to 10% of the data required for a normal image reconstruction. For example, a temporal resolution of 20 to 30 milliseconds can now be achieved achieved, along with an in-plane resolution of 1.5 to 2.0 mm.
While, in principle this technique can be implemented with most current scanners, a practical limitation lies in the amount of computing power required to perform the real-time reconstruction of the images. Improvements in the algorithms used for image reconstruction and the use of parallel computer processors are significantly reducing time required to produce one minute of images displaying motion from one half hour.
The patent literature includes a number of examples of imaging apparatus and image data processing methods for increasing the speed at which images can be reconstructed and for increasing the resulting resolution of the images. For example, a magnetic resonance (MR) imaging apparatus and technique is described as exploiting spatial information inherent in a surface coil array to increase MR image acquisition speed, resolution and/or field of view. Partial signals arc acquired simultaneously in the component coils of the array and formed into two or more signals corresponding to orthogonal spatial representations. In a Fourier embodiment, lines of the k-space matrix required for image production are formed using a set of separate, preferably linear combinations of the component coil signals to substitute for spatial modulations normally produced by phase encoding gradients. The signal combining may proceed in a parallel or flow-through fashion, or as post-processing, which in either case reduces the need for time-consuming gradient switching and expensive fast magnet arrangements. In the post-processing approach, stored signals are combined after the fact to yield the full data matrix. In the flow-through approach, a plug-in unit consisting of a coil array with an on board processor outputs two or more sets of combined spatial signals for each spin conditioning cycle, each directly corresponding to a distinct line in k-space. This partially parallel imaging strategy, dubbed Simultaneous Acquisition of Spatial Harmonics (SMASH), is readily integrated with many existing fast imaging sequences, yielding multiplicative time savings without a significant sacrifice in spatial resolution or signal-to-noise ratio. An experimental system achieved two-fold improvement in image acquisition time with a prototype three-coil array, and larger factors are achievable with then coil arrangements.
In another example from the patent literature, a method of and system are described in which the method comprises acquiring a plurality of magnetic resonance signals from a receiver coil array placed about a subject in the MRI system, where the receiver coil array has a plurality of receiver elements arranged in rows and, during application of a readout gradient in a frequency encoding direction, shifting receiver frequencies by a selectable amount for each row of the array in order to shift a limited field of view in the frequency encoding direction.
In yet another example from the patent literature, an MRI system produces a three-dimensional. Image by acquiring NMR signals that fully sample a central region as a set of asymmetric radial sectors. The NMR signals are acquired with a plurality of receive channels and coils. An image is reconstructed using a homodyne reconstruction combined with SENSE nrocessine.
The patent literature further describes the use of a Halbach magnet array in place of the conventional, very large, horseshow magnet within MRI apparatus. In general, a Halbach magnet array is an arrangement of magnets that augments the magnetic field on one side of the arrangement while canceling the magnetic field, to nearly a zero level, on the other side.
FIG. 1 is a perspective view of MRI apparatus to described in prior art patent documents. The apparatus 10 includes a Halbach array 12 of magnetic structures 14 located at corners of a square, in which probe apparatus 17 is held to receive signals for processing. Each of the magnetic structures 14 includes a dipole magnet, generating a magnetic field extending transverse to its direction of elongation, indicated by arrow 16. Each of the magnetic structures 14 may include one or more permanent magnets, and may additionally or alternatively include one or more electromagnetic coils, which may be held within a tube filled, for example, with liquid helium to achieve superconductivity.
FIG. 2 is a schematic cross-sectional transverse view of the apparatus 10, taken in as indicated by section lines 2-2 in FIG. 1, with an arrow 18 showing a direction of the magnetic field extending transversely within each of the magnetic structures 14. Transverse magnetic fields 20 extend outward from each of the magnetic structures 14, with fields formed in the direction of arrow 22 a central region 24 surrounded by the magnetic structures 14 being strengthened, and with magnetic fields in a region 26 outside the Halbach array 12 being substantially reduced, as reflected in the increased distance between flux lines in the figure.
FIG. 3 is a schematic cross-sectional transverse view of the apparatus 10 showing the magnetic structures 14 oriented so the transverse magnetic fields within the magnetic structures 14 are oriented as shown by arrows 30 to produce a magnetic field in the direction of arrow 32 within the central region 24.
FIG. 4 is an elevation of a hand-held MRI probe 40, described in the patent literature, which includes a coil section 42 having an imaging coil It is noted that the lack of exposure to ionizing radiation during the MRI process makes it possible for an individual to remain with the patient during the process, and that the probe 40 can be held close to the tissues being scanned, with a resulting improvement in the quality of the images being produced.
d to allow a patient, as you described, show what was happening when he had a transient pain or discomfort. The main problem in making such pictures appears to be computer processing time to get the images ready for viewing, since it takes about a half hour to process about a minute of real time film, and limitations on the resolution of the images, which can be expected to limit the resolution of the images. The need for an open structure does not